The use of a columnating assembly for directing a plurality of jets of a temperature controlled gas against the surface of a food product is known. Such apparatus is disclosed, for example, in U.S. Pat. Nos. 3,884,213; 4,154,861; 4,289,792; 4,338,911; and 4,409,453. Notwithstanding the advantages that are achieved through the use of such apparatus, there has remained a need for an impingement heating or cooling apparatus that can effectively and efficiently deliver a flow of temperature controlled gas from a relatively large diameter axial flow fan through a closely spaced columnating assembly that is perpendicular to the axial direction in such manner that the flow of gas is substantially uniform through the orifices distributed across its surface. The direct alignment of an axial flow fan with a columnating assembly is preferred because of the shorter air circulation path that can thereby be obtained. A very short air circulation path makes particularly efficient use of power input to a fan or blower. This enhanced efficiency is multiplied when the unit is used in a refrigerated space, where energy used to drive the blower produces heat which must be offset by increased refrigeration capacity, adding to operating costs.
Large diameter axial flow fans are desirable for use in impingement heating and cooling devices because they permit rapid acceleration of the air mass without having to change the direction of travel of the air mass while passing through the fan. This helps in achieving the high pressure needed to form the columnar jets of temperature controlled gas at a minimal energy cost. Also, by placing the fan as near as possible to the columnating assembly, it is possible to reduce the overall bulk of the unit. This is particularly important, for example, in commercial situations where additional space is either unavailable or prohibitively costly.
However, the use of a large axial flow fan discharging directly against a columnating assembly presents a major design problem in that a cone of low pressure air forms downstream of the fan hub, resulting in a wind "shadow" on the center of the columnating assembly. This low-pressure cone is not a problem in most other axial flow applications, either because the air is fed into a large plenum where pressure equalizes anyway, or because the target of the moving air is far enough downstream to be beyond the tail of the cone and thus out of the "shadow."
Various means have been analyzed and tested in an effort to find suitable apparatus for redirecting high velocity air that is discharged from the fan blade tips back toward the central vortex. Neither flat radial plates nor concentric rings placed in the air stream have proven effective for redirecting air into a uniform pattern over the full surface of a perpendicularly disposed columnating assembly. Although directing vanes have been designed which are severe enough to fill the low pressure space, they have tended to do it at the expense of starving the outwardly disposed orifices of a columnating assembly. Inserting a long cone or a cylinder into the low pressure area or "shadow" can prevent the wasteful counterflow. However, this cone or cylinder effectively blocks any adjacent jets, and is thus not suitable for use in an application where the columnating assembly is situated relatively close, such as for example, less than about two fan diameters from the discharge point.
There also exists a need for an apparatus that can thermally treat a plurality of vertically spaced layers of food products in such manner that each layer of food products is subjected to a progressive thermal treatment cycle whereby it is first contacted with vertically impinging jets of a temperature or humidity controlled gas and is thereafter contacted by a horizontal or alternating horizontal convective flow of such gas.